Electroluminescent displays, such as organic light emitting diode (OLED) displays, are of increasing interest. Commonly, the pixels of such a display are driven using an active matrix of thin film transistor (TFT) circuits, so that each pixel is independently controlled and can be held in an emissive state for substantially greater than 50% duty cycle.
Unfortunately, such displays are as yet imperfect. Displays may suffer from uneven characteristics across pixels, and may also suffer from aging, thus making it difficult to render a high quality image over the desired lifetime of a display.
One source of variability that is of particular interest is the threshold voltage of a driving transistor. A great many circuits and methods for driving pixels have been proposed to address this and other concerns. U.S. Patent Application Publication 2009/0109142 A1, titled “EL DISPLAY DEVICE,” and U.S. Pat. No. 7,876,294, titled “Image display and its control method,” describe several such circuits and methods.
Another reference of particular interest is U.S. Patent Application Publication 2013/0016083 A1 by Maekawa and Miwa, titled “Pixel Circuit, Display Device, and Inspection Method.” This publication discloses a driving circuit and method of operation that provides threshold voltage compensation and also provides the ability to test the pixel driving transistors before completing the manufacture of the electroluminescent elements of each pixel. An embodiment is shown using n-channel TFTs.
P-channel TFTs are commonly used in the display industry today, although n-channel TFTs can also be fabricated. As organic TFT technology develops, p-channel TFT technology may be preferred, since the materials and processing are different for p-type and n-type materials, and may continue to favor p-type materials over n-type as they do today.
Maekawa also discloses at [0064] a p-channel embodiment where the electroluminescent element is connected on the source side of a p-channel driving transistor, i.e. between positive power supply VCC and the source of the p-channel driving transistor. The Maekawa p-channel embodiment thus requires connecting the source of the driving transistor to the cathode of the electroluminescent element.
Because of the processing temperatures involved, it is common to form TFT layers on a substrate prior to deposition of, for example, delicate organic layers of an OLED electroluminescent element. Within the OLED, indium-tin-oxide (ITO) is a popular anode electrode material. Although lower temperature ITO processes are becoming available, there is still a preference in the display industry to deposit ITO with higher temperature processes that must be performed before OLED deposition.
Another approach to fabricating Maekawa's p-channel embodiment would be to fabricate the OLED in a conventional order, with an anode at the bottom and closest to the TFT layers, and a cathode at the top. Then the connection from driving TFT to the cathode could be made using vias. However this requires additional process steps.
Thus, there is still a need for a p-channel driving circuit and method for an electroluminescent display that compensates for threshold voltage variations, and offers an inspection capability before manufacture of electroluminescent elements has been completed.